1. Field of the Invention
The present invention relates to an electrostatic relay and, in particular, to an electrostatic micro relay activated by an electrostatic force.
2. Description of the Related Art
An electrostatic micro relay is a super micro relay made by using a technique for processing a semiconductor and is used for alternating electrical signals or high frequency signals. The micro relay contacts and separates a moving contact with and from a fixed contact, by utilizing electrostatic attraction. As types of such micro relays, a cantilever type in which a moving electrode having a generally plate shape is supported at only one end thereof, a parallel plate type in which the moving electrode is movably supported at both ends thereof, or a comb type in which each of a moving and a fixed electrodes has a comb-shape capable of engaging with each other, are possible.
In the electrostatic relay, the electrostatic attraction force is in proportion to a square of the voltage between a fixed electrode and a moving electrode and is in inverse proportion to a square of the distance between the electrodes. Therefore, in order to reduce a required voltage for activation, the distance between the electrodes must be short. In this case, it is difficult to increase a contact gap between a fixed contact of the fixed electrode and a moving contact of the moving electrode. However, it is very important to obtain a large contact gap in the design of the micro relay, because a large contact gap is preferably for restraining discharge between the electrodes and leakage of the high frequency signal.
In this regard, Japanese Unexamined Patent Publication (Kokai) No. 9-251834 discloses an electrostatic relay including a fixed electrode and a moving electrode, both of the electrodes having comb-shaped structures, in order to lengthen the contact gap. In this electrostatic relay, the moving contact is movable in parallel with a substrate so as to contact or separate with or from the fixed contact. The electrostatic attraction force between the contacts may be relatively large, due to the comb-shaped structure of the electrodes, whereby the contact gap may be extended.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2002-289081 discloses an electrostatic micro relay having a movable substrate supported at two points symmetrically positioned in relation to a moving contact. This relay has a protrusion capable of contacting a substrate in order to increase a contact opening force and to stabilize the contact and the separation between two contacts.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2002-326197 discloses a MEMS element having a movable substrate and a stopper or a protrusion arranged on the substrate by which a repulsive force of a spring supporting the substrate may change nonlinearly.
In the comb-type relay, as described above, the electrostatic attraction force may be increased by enlarging the surface areas of the comb-shaped electrodes opposing each other. Therefore, a contact gap between the electrodes may be increased. However, in the relay described in Japanese Unexamined Patent Publication (Kokai) No. 9-251834, for example, as the moving electrode may move only in a horizontal direction, the moving contact abuts to the fixed contact such that each abutting point of the two contacts is always the same. Therefore, in particular when the contact and the separation of the contacts are frequently repeated, only the same point of each contact is worn and damaged, whereby the life of each contact becomes shorter and the contacts must be exchanged more frequently.
On the other hand, in the parallel plate type relay, the moving electrode or a movable plate may be moved perpendicular to a surface while the movable plate is parallel to the fixed electrode. Therefore, when a spring constant of a spring supporting the moving electrode is relatively low, the contact gap may be increased and the electrode may be moved by a relatively low electrostatic force. However, when the spring constant is low, a mechanical malfunction due to mechanical vibration or external noise may occur, further, sticking or adhesion between the contacts may be caused as the opening force for separating the moving contact from the fixed contact becomes smaller. Therefore, a large contact gap is not allowed. On the other hand, when the spring constant is high, a high stress is applied to the spring on contacting or separating the moving contact with or from the fixed contact, whereby the life of the spring becomes shorter.
The cantilever type relay has a simple configuration and is easily made, however, this type of relay requires a larger electrostatic force than that of other types of relay having the same contact gap. Therefore, it is difficult to obtain the large contact gap in the cantilever relay. Further, a high stress is applied to the cantilever on contacting or separating the moving contact with or from the fixed contact, whereby the life of the cantilever or the moving contact becomes shorter. Some cantilever-type relays have been developed to realize a large contact gap, however, each of these relays has a complicated electrode configuration and a high production cost.